The objective of the Multicenter Study of Hydroxyurea (MSH) Follow-up is to establish a follow-up study of the adult patients who participated in the Multicenter Study of Hydroxyurea in Sickle Cell Disease (MSH) in order to ascertain the long term toxic effects of hydroxyurea usage in this patient population. Prior to the MSH, hydroxyurea had never been used at this dosage in this patient population. The patients will be followed annually to determine health status, quality of life, incidence of malignancies, and birth defects in their offspring. in addition, mortality rates will be determined so that a comparison can be made between this cohort and the mortality data from the Cooperative Study of Sickle Cell Disease (CSSCD) adult cohort and the normal African-American population mortality data.